The present invention relates to a belt-driven automotive engine accessory drive system and means for tensioning such a system. Drive systems for the front end accessories of automotive engines typically include a belt having a tensioning device for maintaining the belt in contact with all the pulleys of the system, including the drive pulley, which is usually attached to the crankshaft of the engine, as well as with a plurality of driven pulleys, with at least one driven pulley attached to each rotating accessory. Such accessories frequently include an alternator, a power steering pump, an air conditioning compressor, a secondary air pump for emission controls, as well as other types of rotating devices.
Conventional tensioners utilize elastic force provided by, for example, a flat wire spring for maintaining a tensioning pulley in contact with the drive belt. Such a pulley is shown as item No. 34 in FIG. 1 of the present application. Although damped tensioners have been used to some extent in automotive front end accessory drive systems, such tensioners typically are symmetrical in their damping characteristics. In other words, motion of the tensioner is damped both in the direction toward the belt and away from the belt. Unfortunately, if the tensioner is set up with a fairly low damping rate so as to allow the tensioner wheel to be compliantly in contact with the belt, the tensioner will be allowed to pull away from the belt in the event that the following series of events occurs within the accessory drive system.
FIG. 6 illustrates a problem with conventional tensioners which is solved by a tensioner according to the present invention. Operation of a front end accessory drive system with a corrective tensioner according to the present invention is shown in FIG. 7. Both plots illustrate the rotational speed or angular velocity of an engine's alternator, idler pulley, and crankshaft pulley. The rotational speed of the idler pulley is a direct indicator of the speed of the drivebelt because it is assumed for the purpose of this discussion that minimal slip occurs between the idler pulley and the drivebelt; this is a good assumption because the rotating inertia of the idler pulley is relatively slight as compared with the rotational inertia of the other components of the engine's front end accessory drive system, particularly the alternator. As shown in both plots, crankshaft rpm decreases at a very high rate in the situation being considered. It has been determined that during wide open throttle transmission upshifts at lower gear speeds, such as the upshift from first to second gear with an automatic transmission and an engine speed of, for example 4500 rpm, the crankshaft may decelerate at a rate approaching 20,000 rpm per second. These high deceleration rates cause the front end accessory drivebelt to slip on one or more pulleys, particularly the crankshaft pulley, thereby giving an objectionable squealing noise which will be audible to the driver of the vehicle. The squealing noise produced by the loose drivebelt slipping on the crank pulley is caused by an overrunning effect of the alternator. FIGS. 6 and 7 show rotational speed data produced during tests in which an instrumented engine was rapidly decelerated from a high rate of speed. FIG. 6 illustrates the behavior of a prior art system; FIG. 7 illustrates a system according to the present invention. As shown in FIG. 6, alternator speed tails off to zero at about 300 msec. after the crankshaft stops. Similarly, the idler rpm and drivebelt speed tail off to zero at about 200 milliseconds following the stopping of the crankshaft. This occurs because once the crankshaft stops, the high rotational inertia of the alternator causes it to remain rotating and causes the alternator to pull the tensioner away from the belt. In turn, this causes a "bubble" of belt to extend from the alternator to the crankshaft pulley, and as a result the drivebelt slips on the crankshaft pulley. The resultant squeal may be very audible. In contrast with the operation according to the conventional tensioner at FIG. 6, FIG. 7 shows the results of the use of a tensioner and control system according to the present invention. In essence, the tensioner has a governor for controlling the rotational motion of the tensioner arm such that the tensioner's arm will be freely able to rotate toward the drivebelt, while movement of the arm and tensioner wheel away from the drivebelt is resisted. Because the tensioner cannot move readily in the direction away from the drivebelt, the tension within the belt is maintained and, as a result, the deceleration rates of the drivebelt, the alternator and the crankshaft converge. This is shown graphically in FIG. 7. Note that the three plots for alternator, idler and crankshaft all converge at a about 1100 msec. This means effectively that the alternator is no longer permitted to pull the tensioner away from the belt, and as a result, the alternator is decelerated in close congruence with the crankshaft's deceleration. This has the beneficial effect of preventing squeal of the drivebelt at the crank pulley, because with the tension maintained at a proper level in the drivebelt, the belt will not slip at the crankshaft pulley.